The fish swimbladder is a gas-filled organ with many structural and functional similarities to the mammalian lung. Although the mammalian lung cannot tolerate oxygen concentrations substantially higher than that of atmospheric air, the swimbladder has adapted to a wide range of oxygen concentrations (1-99%). Superoxide dismutase, catalase, and peroxidase are present in swimbladder tissue and the possible role of these enzymes in protecting the swimbladder against oxygen toxicity will be investigated. Enzyme activity will be measured biochemically and enzymes will be located ultrastructurally by cytochemical techniques. Mammalian lung will be studied by the same techniques and comparison of the antioxidant defense system will be made between lung and swimbladder. Different types of fish are adapted to different oxygen concentrations in the swimbladder. Fishes adapted to low, moderate, and high oxygen concentrations will be compared. If superoxide dismutase, catalase, and/or peroxidase are protecting the swimbladder from oxygen, enzyme activity should be related to oxygen concentration. Increased cholesterol resulting from hyperoxia has been reported in the mammalian lung. The effect of high oxygen concentration on cholesterol content of the swimbladder will also be investigated by cytochemical and autoradiographic techniques.